Ethylene and propylene refrigeration compressors are commonly used in refineries for gas separation operations. Control and protection of these compressors is not only among the most complex of control applications, but a challenge fraught with inaccuracies. However, it is also an application where the economics of good control is large. Although energy savings can be profitable, an important objective is to maximize production while minimizing process upsets, surge, and shutdowns.
Most refrigeration systems are multistage machines with sidestreams entering and/or exiting between the stages. Since flow measurement devices are not available between stages, and the gas temperature entering most stages is unknown, it is impossible to calculate reduced flow reliably.
The method commonly used today requires the assumption be made that temperatures into the separate stages do not change considerably. When temperatures do change, the safety margin must be increased to protect the compressor under worst-case conditions.